1. Field of the Invention
The present invention generally relates to a hydrophilic oxygen permeable contact lens and a method for producing the same. More particularly, the invention is concerned with such a hydrophilic contact lens whose appearance is not deteriorated by a surface treatment for a high degree of hydrophilic property at the lens surface, and which contact lens is capable of maintaining the high hydrophilic property for a long time, even after repeated washing or cleaning of the contact lens by a flow of water. The invention is also concerned with a method for producing such a hydrophilic contact lens.
2. Discussion of the Prior Art
There have conventionally been used non-water swellable or absorbable contact lenses, such as a hard contact lens made of a polymer consisting principally of methyl methacrylate, an oxygen permeable contact lens made of a polymer consisting principally of siloxanyl methacrylate and/or fluoroalkyl methacrylate, for instance, and a non-water swellable or absorbable soft contact lens formed of a silicone rubber made mainly of polydimethylsiloxane, or a butyl rubber made mainly of butyl acrylate. The surface of these non-water absorbable contact lenses is hydrophobic. In particular, the oxygen permeable contact lens which has recently attracted public attention exhibits a high degree of hydrophobic property. For improving oxygen permeability of a contact lens, there is a general tendency that a material for forming the contact lens includes an increased amount of silicon- or fluorine-containing component. In other words, a contact lens which exhibits a higher degree of oxygen permeability contains a comparatively large amount of silicon- or fluorine-containing component. Accordingly, the surface of the thus-formed contact lens having the higher degree of oxygen permeability exhibits a considerable high degree of hydrophobic property.
Generally, the above-described oxygen permeable contact lens is transported or handled while it is immersed in a storing or preserving solution. In this case, bacteria would be generated in the storing solution, or the storing solution would leak in the process of manufacture or transportation of the contact lens. To solve this drawback, it is considered to handle or transport the contact lens without being immersed in the storing solution, in other words, to keep the contact lens in a dry state. However, if the oxygen permeable contact lens is kept in the dry state for a relatively long period before it is worn on a human eye, the lens would tend to be excessively dry due to its low degree of hydrophilicity. Accordingly, the contact lens which has been kept in the dry state as described above tends to blur a vision of a lens wearer when the lens is initially worn on the eye of the wearer, and a tear fluid is readily repelled on the surface of the contact lens, whereby the eyesight of the lens wearer is apt to be unstable.
For solving the above problems, there have been made various attempts to improve the hydrophilicity of the oxygen permeable contact lens. For example, JP-A-4-104220 discloses a technique for coating a surface of a contact lens with a hydrophilic compound. Described more specifically, a water-soluble polymer having an amino group is crosslinked by a suitable crosslinking agent, and is then applied to the lens surface to give a hydrophilic coating on the lens surface. However, the contact lens whose surface is coated with the hydrophilic compound according to the above method is not capable of maintaining the hydrophilic property at a satisfactory value for a long time, and the hydrophilic property of the lens is easily deteriorated when the contact lens is repeatedly washed or cleaned by a flow of water.
As another attempt to improve the hydrophilicity of the oxygen permeable contact lens, it is proposed that the oxygen permeable contact lens be subjected to a plasma surface treatment. However, the contact lens which has been subjected to such a plasma surface treatment would not exhibit a satisfactory hydrophilic property at its lens surface if the contact lens is kept in a dry state for a relatively long period before it is worn by the user. In this case, a layer formed on the lens surface as a result of plasma treatment undesirably enters into the composition of the contact lens. Thus, it has been long desired to develop an oxygen permeable contact lens which assures excellent durability in its hydrophilic property at its lens surface.
It is also proposed to effect plasma polymerization of a hydrophilic monomer for improving the hydrophilicity of the contact lens. According to this proposal, there is formed a plasma-polymerized layer on the surface of the lens, whereby the contact lens exhibits improved hydrophilicity. However, it is generally difficult to control the formation of the plasma-polymerized layer on the lens surface. For instance, the lens surface tends to be plasma-polymerized to an excessive extent for attaining the desired degree of hydrophilicity at the lens surface. As a result, the contact lens is unfavorably colored, or a residual monomer irritates the eye of the lens wearer. Thus, the contact lens produced according to this method is not suitable for use.